Summary Air Canada flight ACA3578, an Airbus A320, departed Calgary, Alberta, en route to Toronto, Ontario, at flight level (FL)370. Northwest Airlines flight NWA853, a Boeing757, departed Detroit, Michigan, en route to Anchorage, Alaska, at FL350. Both flights were under radar control. NWA853 requested and received clearance to climb to FL370 approximately 43nautical miles (nm) southeast of the Red Lake, Ontario, VOR (VHF omnidirectional range). When NWA853 was approximately 35nm southeast of the Red Lake VOR, the air traffic controller noticed that the two flights were at the same altitude, travelling toward each other on reciprocal tracks. He issued instructions to reestablish separation, but the two aircraft passed with 1.5nm horizontal and 600feet vertical spacing. The required minimum separation was 5 nm horizontally or 2000feet vertically. Ce rapport est galement disponible en franais. Other Factual Information Winnipeg Area Control Centre (ACC) is organized using a number of specialties, each responsible for a portion of the airspace comprising the Winnipeg Flight Information Region. Saskatchewan, Ontario, and Winnipeg are three of these specialties. Each specialty is divided into sectors, each of which is responsible for a portion of the airspace. On the evening of the occurrence, the Winnipeg specialty had four sectors open: East low-level, West low-level, Gimli high-level, and Dryden high-level. The occurrence took place in the Dryden high-level sector. Winnipeg ACC was undergoing airspace restructuring as part of a national airspace reorganization plan. With airspace restructuring, new controllers were being qualified in only low- or high-level airspace operations, but not both, while some controllers who were trained before the restructuring were still dual-qualified in low- and high-level airspace. Nav Canada management and the Canadian Air Traffic Control Association agreed that dual-qualified controllers would normally work in only low- or high-level sectors during a shift, but not both. This agreement also included a provision that permitted team supervisors to work low- and high-level sectors during a shift. Because new controllers were not dual-qualified and because dual-qualified controllers normally did not work both low- and high-level sectors during a shift, staffing requirements were elevated. However, the number of available controllers did not increase correspondingly. Staffing requirements were expected to be temporarily increased only during the restructuring phase and were expected to return to normal when the restructuring was completed. Consequently, Winnipeg ACC was understaffed. The shift schedule for the Winnipeg specialty required nine controllers on the evening of the occurrence: four in the high-level sectors, four in the low-level sectors, and one as the team supervisor. Only seven qualified controllers were on duty; the specialty was missing one low-level controller and the team supervisor. One high-level controller, who was dual-qualified was designated as the acting team supervisor, enabling him to work low- and high-level sectors during the shift. The shift manager expected that the acting supervisor would be controlling and did not expect him to perform any supervisory duties. The acting team supervisor also understood that he was expected to be controlling and not perform any supervisory duties. Therefore, the specialty had three controllers working low-level sectors, three working high-level sectors, and the acting team supervisor working either low- or high-level sectors, as required, during the shift. The Saskatchewan and Ontario specialties were also understaffed: Saskatchewan by four controllers and Ontario by three controllers. Winnipeg ACC traffic capacity was reduced because of staff shortages, and air traffic flow management restrictions had been implemented. These restrictions required the Winnipeg specialty controllers to provide the Ontario and Saskatchewan specialties with 20 nautical miles (nm) longitudinal separation between aircraft at the same altitude on similar tracks, and to deny aircraft direct routings or clearance to fly at altitudes inappropriate for the direction of flight. The two flights involved in the occurrence were controlled by the Dryden sector controller, who was working the radar and data positions of the sector. It is an accepted and common practice for a controller to work radar and data positions simultaneously. Factors considered when deciding whether a controller will work both positions are traffic conditions, controller experience and capabilities, controller workload, and available staff. Traffic was assessed as heavy, with high complexity and high workload. Twenty minutes before the occurrence, the controller had moved from one control console to another so that maintenance could be carried out on the first console to correct a display problem. All necessary equipment was serviceable at the console to which he moved. When the Dryden controller moved, a second controller working at another console asked him whether he needed someone to work the data position. The Dryden controller stated that he would get established at the new console, then assess the need for a data controller. Once established at the new console, he became absorbed in his control duties and did not assess the need for assistance. At the time of the occurrence, two of the three high-level controllers in the Winnipeg specialty were working the two high-level sectors; the third high-level controller was on a break. The acting team supervisor was working a low-level sector. Standback supervision was not provided. The Dryden controller had 21 years experience, with 7 years as an instrument flight rules controller. He was dual-qualified and scheduled to work only high-level airspace on the day of the occurrence. The controller was qualified for radar and data positions. He was on his second day of work after four days leave and five days off. He had been on duty for seven hours since the beginning of his shift and had taken a 15-minute rest break before taking over the Dryden sector. He had been working the Dryden sector for about one hour at the time of the occurrence. At 2012:24 central daylight time,1 the Dryden controller agreed with the sector to the east to have an eastbound aircraft descend from flight level (FL)390, an altitude inappropriate for direction of flight; at 2014:00, he cleared it to descend to FL370, an appropriate altitude. At 2024:07, he denied a request from a westbound aircraft for a direct routing to a position well to the west, mentioning the flow restrictions as the reason for the denial. These two control actions indicate that the controller was aware of air traffic flow management restrictions. ACA3578 was at FL370 eastbound toward the Red Lake VOR (VHF omnidirectional radio range) and was cleared after Red Lake to Sioux Lookout and then to the Marathon VOR, as was shown on the flight progress strip. At 2023:35, ACA3578 was about 10nm west of Red Lake and requested clearance to proceed direct to Marathon. The controller told the aircraft to stand by. At 2024:15, NWA853 was at FL350 and about 45nm southeast of Red Lake, between Sioux Lookout and Red Lake, flying northwest toward Red Lake. NWA853 told the controller that they were experiencing turbulence and requested clearance to climb to FL370. The climb was requested for passenger comfort rather than for safety of flight. The controller confirmed with ACA3578 that there was no turbulence at FL370 and used projected track lines (PTLs) from the targets of ACA3578 and NWA853 on his radar indicator module (IM) to make a traffic conflict assessment. PTLs are updated with each radar scan and are based on current track and ground speed; the track projection will not display an impending flight-planned turn. At that time, ACA3578 was still flying eastward toward Red Lake, and the PTLs displayed a projected track for ACA3578 eastward past Red Lake. The controller did not look at the flight progress strip for ACA3578 to confirm its routing after Red Lake. Based on the PTLs, he assessed that adequate spacing between the two flights would be maintained. At 2024:30, the controller cleared NWA853 to climb to FL370. From 2024:58 until 2026:30, the Dryden controller coordinated the NWA853 altitude change with sectors to the west and north. He also received approval from the sector to the east to clear ACA3578 direct to Marathon. At 2026:25, NWA853 levelled off at FL370. Canadian Aviation Regulation (CAR) 602.34 requires that aircraft operate at altitudes appropriate to the direction of flight, unless another altitude is assigned by air traffic control (ATC). Air Traffic Control Manual of Operations (ATC MANOPS) 432.2.C permits controllers to assign an altitude inappropriate to the direction of flight if a pilot requests it because of turbulence, provided that the pilot informs the controller of the time at which the flight can be cleared to an appropriate altitude and provided that the altitude has been approved by the affected sectors. After NWA853's request for FL370, an altitude inappropriate for the direction of flight, the crew indicated that they could accept FL390, the next higher appropriate altitude, in about 30 minutes. Controllers of the affected sectors to the west and north approved the wrong-way altitude. ATC MANOPS 432.3 requires that, when applying section432.2.C, a controller must issue radar vectors or offset tracks to establish an aircraft at least 5nm from the centreline of the airway. This procedure is intended to maintain controller involvement and provide an additional margin of safety while aircraft are operating on wrong way altitudes. The controller did not issue any instructions to offset NWA853 from the airway. When a cruising altitude inappropriate to the aircraft track is assigned, ATC MANOPS 432.6 requires controllers to post warning indicators by circling the altitude in red on the flight progress strip. The altitude 370 on the flight progress strip for NWA853 was circled in red. Air traffic controllers issue clearances and instructions to pilots to achieve control objectives. Pilots are required to comply with all clearances received and accepted by them and are required to comply with all instructions directed to and received by them. Clearances are authorizations; instructions are directives. At 2026:35, just as the controller was beginning to issue a direct-to-Marathon clearance to ACA3578, he saw on the IM that the two flights were on reciprocal tracks at the same altitude. He immediately issued instructions to ACA3578 to turn left 15 and cleared NWA853 to maintain FL350. ACA3578 turned as instructed; NWA853 questioned the controller about the descent clearance and did not immediately descend. The controller made a second more direct, but ambiguous, transmission instructing NWA853 to descend, followed by a third, very clear and urgent transmission instructing them to descend. At 2027:18, after the third transmission, NWA853 commenced a descent. Pilots and controllers form mental pictures of the relative positions of aircraft to assist them in understanding an overall traffic situation. When involved in a communication exchange, pilots and controllers usually process received information using mental expectations that seem most appropriate for the activity they are performing at the time of the communication. If a message is unexpected or unusual, the mental expectation held by the information receiver may hinder understanding of the message and could delay a response to new information. NWA853 had been level at FL370 for only 18seconds when the controller cleared NWA853 to descend to FL350; the crew was not aware of oncoming traffic and was not anticipating a descent clearance. ATC MANOPS 507.1 instructs controllers to issue a safety alert to an aircraft if you are aware the aircraft is at an altitude which . . . places it in unsafe proximity to . . . another aircraft. The specific phraseology used in such a situation is traffic alert (position of traffic, if time permits), advise you turn right/left (specific heading, if appropriate), or climb/descend (specific altitude, if appropriate) immediately. The Dryden controller did pass traffic information to both flights but did not use ATC MANOPS 507.1 safety alert phraseology to emphasize the urgency of the situation. In 1997, TSB issued Aviation Safety Advisory 970038 suggesting that Nav Canada may wish to consider additional means of emphasizing the special need for standard safety alert phraseology. This safety advisory followed an investigation of a number of occurrences in which controllers did not use safety alert phraseology specified in ATC MANOPS 507.1 in situations that should have evoked a sense of urgency. Nav Canada took action by issuing Air Traffic Services Bulletin 9801 about safety alert phraseology and by making this subject a mandatory portion of refresher training given to controllers in 1998/1999. The occurrence controller received this refresher training in February 1999. Both aircraft involved were equipped with traffic alert and collision-avoidance systems (TCASs); CARs do not require TCAS to be installed in aircraft flying in Canadian airspace. TCAS is designed to operate independently of ATC and provides traffic alerts (TAs) and resolution advisories (RAs) to a flight crew. TAs are intended to help crews see conflicting traffic and to alert them to the possibility of an RA. RAs warn crews of potential collisions and provide descent or climb commands to enable them to avoid an intruder aircraft. TCAS uses a datalink between aircraft transponders to provide complementary RAs (one aircraft climbing and one aircraft descending). About 20 seconds after the controller identified the conflict and issued instructions, both flight crews received TCAS TAs. About 20 seconds after the TAs, both crews were following the controller's instructions when they received RAs. ACA3578 received an RA to descend, and NWA853 received an RA to climb; the controller had instructed NWA853 to descend. Each crew independently decided to disregard the RA commands because they were each aware of the controller's instructions to the other crew and were each already following the controller's instructions. CARs do not compel pilots to comply with RA commands. Air Canada's flight operations manual states that compliance with TCAS RESOLUTION ADVISORY commands is mandatory, unless, in the opinion of the Captain, doing so would compromise the safe operation of the flight, or unless the flight crew has better information . . . about the intruder causing the RA. The Northwest Airlines flight operations manual states that compliance with RA guidance is mandatory to ensure safe separation. Northwest Airlines Boeing757 Operating Procedures state that during the RA, pilots must maintain situational awareness . . . TCAS is simply another tool' to help reduce the collision potential. CARs require flight crews to report a deviation from a clearance in response to a TCAS RA; however, no report is required if there is no deviation in response to an RA. Neither flight crew reported a RA to the controller. The minimum spacing between the flights occurred at 2027:42; NWA853 could see ACA3578, although ACA3578 never did see NWA853. At 2027:47, the controller cleared ACA3578 to proceed direct to Marathon. NWA853 levelled off at FL350 at 2028:44. The acting team supervisor was working at the Winnipeg West low-level sector and was alerted to the occurrence by a change in the Dryden controller's voice. He instructed the controller working the Winnipeg East sector to monitor the West sector and left his position to inform the shift manager of the occurrence. By the time the shift manager and the acting team supervisor arrived at the Winnipeg specialty, the occurrence controller was handing over the Dryden sector to a controller who had just arrived from the break room. The occurrence controller was removed from control duties once the sector handoff had been completed. The original performance specifications for the ATC radar data processing system (RDPS) software included provisions for aircraft conflict alerting. During testing, the RDPS conflict-alert function was found to have several faults and was not considered acceptable for operational use. This function was still not operational at the time of this occurrence. In the investigation report on occurrence A99H0001, involving a loss of separation between two Boeing 767 aircraft west of Langruth, Manitoba, TSB made a recommendation for the consideration of both Nav Canada and Transport Canada that: Nav Canada commit, with a set date, to the installation and operation of an automated conflict prediction and alerting system at the nation's air traffic control facilities to reduce the risk of a midair collision.